Means and method for controlling load turndown in a fluidized bed combustor

ABSTRACT

A means and method of controlling the load turndown in a fluidized bed combustor including a fluidized combustion bed having an independent fluidization air source. A separately fluidizable heat transfer bed surrounds the fluidized combustion bed. By independently adjusting the fluidization of the heat transfer bed, the load turndown of the fluidized bed combustor can be accurately and efficiently controlled.

This is a continuation of application Ser. No. 162,278 filed 2-29-88 andnow abandoned, and that application is a continuation of applicationSer. No. 907,110 filed 7-15-86 and now patent no. 4,762,090.

BACKGROUND OF THE INVENTION

a. Field of the Invention.

This invention relates to fluidized bed combustors, and in particular, ameans and method for controlling the load turndown ratio in a fluidizedbed combustor.

b. Problems in the Art.

Combustion of fuel in a fluidized bed is a conventional and well-knownprocess. The advantages of fluidized bed combustion include, but are notlimited to, high rate of heat transfer between the hot bed materialcontaining the burning fuel and the wall enclosing the bed, the abilityto handle a wide variety of fuel types, and the ability to enactpollution control and treatments.

A significant problem exists, however, with fluidized combustion bedsystems. While heat transfer capabilities between the combusting fueland the wall of the combustion bed are improved by the fluidizationmaterial, the ability to control the rate of heat transfer, called theload turndown for the bed, is very limited.

In many cases, the fluidized bed combuster is utilized to produce heator steam. Depending on the use, it is many times desirable to be able toclosely control and vary the combustor output, which is controlled bythe heat transfer rate or load turndown. While an obvious method ofcontrolling turndown ratio is to increase or decrease combustion in thefluidized bed, which would produce a corresponding increase or decreasein heat, doing so can have adverse effects on both combustion efficiencyand pollution emissions, both of which are primary parameters in mostcombustion environments.

A variety of methods have been attempted to allow control of load turndown without deleterious effects to combustion efficiency and pollutioncontrol. As discussed above, if bed temperature is changed, efficiencyis compromised as well as pollution control. Furthermore, temperaturechange itself is limited by combustion processes themselves.

Another method varies the fluidization air in different sections of thefluidization bed to produce partial bed slumping which in turn reducesactive in-bed heat transfer areas. Primary problems with this method arethat air leakage to the slumped side causes agglomeration, and splashcauses bed material transfer to the slumped side and makes anyrestarting in that side difficult.

The method of reducing fluidized bed height by reducing the velocity offluidization air (called "velocity turndown") results in erosion oftubes in the splash zone, and a bed height change is insufficient forlarge margin turndowns.

Reducing in-bed heat transfer area by actually discharging some of thebed materials also results in erosion of tubes in the splash zone andpresents very troublesome particles handling problems.

Independent fluidization or aeration of a separated bed section (calleda "wing panel") is insufficient because the heat transfer coefficient isinsensitive or too insensitive to such velocity change, and this systemis limited to small combustion units.

The prior methods of controlling load turndown in a fluidized bedcombustor have many practical difficulties and can provide only partialcontrol of load turndown.

It is therefore a primary object of the present invention to provide ameans and method for controlling load turndown in a fluidized bedcombustor which improves over or solves the problems and deficiencies inthe art.

Another object of the invention is to provide a means and method forcontrolling load turndown in a fluidized bed combustor which allows awide variation in load turndown without adverse effects on combustionefficiency and pollution emissions.

A further object of the invention is to provide a means and method forcontrolling load turndown in a fluidized bed combustor which allowscontrol of the load turndown independent from control of the combustionof the fluidized bed combustor.

Another object of the invention is to provide a means and method forcontrolling load turndown in a fluidized bed combustor which is simple,efficient, and precisely controllable.

These and other objects, features, and advantages of the invention willbecome apparent with reference to the accompanying specification anddrawings.

SUMMARY OF THE INVENTION

The present invention utilizes a conventional fluidized combustion bed.The fluidized combustion bed is comprised of and bounded by a containerwhich contains the fuel, bed material, and fluidization air whichcombine during the combustion process. A second container defining asecond fluidization bed called a fluidized heat transfer bed, surroundsthe container defining the fluidized combustion bed.

The fluidized combustion bed and fluidized heat transfer bed each haveindependent and separately controlled fluidization air inputs so thatdifferent degrees of fluidization can be achieved in each. The exhaustoutput from the fluidized combustion and heat transfer beds can beeither separately outletted, or combined. If combined, this exitingfluidized air can be used as secondary air for combustion.

The load for the fluidized bed combustor can be, but is not limited to,a water jacket which directly surrounds the fluidized heat transfer bed.It is to be understood, however, that other known in the art loads canbe used in combination with the present invention.

Load turndown can be precisely controlled over a wide variation byinstigating and maintaining combustion in the fluidized combustion bed,as is well known in the art, utilizing the separate fluidization inputair to maintain efficient combustion with minimal pollution. By varyingthe fluidization air level entering the fluidized heat transfer bed fromits separate source, the rate of heat transfer from the fluidizedcombustion bed to the load can be accurately controlled, all withoutloss in efficiency of combustion or increase in pollution emission.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of one embodiment of theinvention.

DETAILED DESCRIPTION OF THE PREFRRED EMBODIMENT

In reference to FIG. 1 of the drawings, there is shown schematically afluidized bed combustor 10 according to the invention. A fluidizedcombustion bed is defined by container 12 having vertical walls 14 and abottom fluidization air distributor plate 16. Fluidization air issupplied through air plenum 18 which is connected to an independentlycontrollable fluidization air source 20.

The contents of container 12 for the combustion bed are a combination ofbed material such as sand or other granular material known in the art(reference numeral 22) and fuel, for example coal (reference numeral24). Fuel is inputted into container 12 by means conventional and knownin the art, and which are not shown in FIG. 1. Container 12 isconstructed to retain the bed material 22 and fuel 24 during combustionand over the range of fluidization air pressure levels for the fluidizedcombustion bed.

In FIG. 1, container 12 is cylindrical in shape. A second annularcontainer 26 surrounds container 12. Annular container 26 hascylindrical vertical walls 28 and an annular air distributor plate 30which extends between vertical walls 28 and vertical walls 14 ofcontainer 12.

A second air plenum 32 channels fluidization air from an independent,separately controllable fluidization air source 34 into annularcontainer 26 which contains only bed material 36 (sand or other granularmaterial).

Both container 12 and annular container 26 open to a common exhaustoutlet 38. Air exiting the heat transfer bed serves as secondary air forcombustion by mixing it with the combustible gas exiting the combustionbed. After combustion is completed, the gas mixture is removed atexhaust outlet 38.

Directly adjacent to and annularly surrounding vertical walls 28 ofannular container 26 is an annular water jacket 40 containing water 42.Water 42 in annular jacket 40 receives heat from the combustion bedtransferred through the heat transfer bed and is transformed into hotwater or steam for further use.

Operation of the invention is as follows. Fuel is introduced and ignitedin the combustion bed to initiate heat producing combustion.Fluidization air is introduced into the combustion bed through airplenum 18 to fluidize the combustion bed to promote combustion and toenhance heat transfer from the burning fuel to bed material 22 andultimately to vertical walls 14. This heat transfer is at a high andefficient rate as is known with fluidized bed combustors.

Combustion in the combustion bed is controlled by introduction of fueland fluidization air to produce the level of combustion desired and themost efficient combustion possible consistent with minimized pollutionemissions.

The amount of heat allowed to be transferred to the water jacket is thencontrolled by the independent control of fluidization air through airplenum 32 to the heat transfer bed.

Overall heat transfer rate to water jacket 40 is determined by the heattransfer coefficients associated with the combustion bed and the heattransfer bed and water jacket 40. In the present invention, only theheat transfer coefficient associated with the heat transfer bed ismanipulated to control the overall heat transfer rate, that is, theboiler load. If no air is passed through the heat transfer bed, then theunfluidized bed has the poor heat transfer characteristics of packedgranular material. If only sufficient air is passed through the heattransfer bed to minimally fluidize the bed, then increased heat transferdue to convection occurs. Finally, if the heat transfer bed is violentlyfluidized, enhanced heat transfer characteristic of a bubbling fluidizedbed occurs. A continuous and large variation in heat transfer rate canbe produced in going from unfluidized to violently fluidized states.

The load turndowns that can be achieved depend on the radial dimensionof the heat transfer bed, and the type of material which is used in theheat transfer bed. For a typical one inch radical dimension heattransfer bed, the ratio of maximum to minimum load is ten.

The included preferred embodiment is given by way of example only, andnot be way of limitation to the invention, which is solely described bythe claims herein. Variations obvious to one skilled in the art will beincluded within the invention defined by the claims.

For example, the preferred embodiment of FIG. 1 depicts "fire-tube"boiler construction in which the water is contained in a large pressurevessel. Alternative boiler designs, such as "water-tube" construction inwhich the water is contained in an array of small diameter tubes, couldalso be utilized in accordance with the invention.

Many different conventional known in the art methods and apparatus canbe used to introduce fuel for combustion into container 12.

Different configurations for air plenums 18 and 32, as well as separateexhaust outlets for each fluidized bed, could be utilized.

Additionally, different furnace loads can be used, such as are known inthe art.

What is claimed is:
 1. A method of controlling the load turndown in afluidized bed combustor, the load comprising one or more chambers havinga fluid, comprising the steps of:combusting fuel in a fluidizedcombustion bed; surrounding the fluidized combustion bed with aseparately fluidized heat transfer bed that is free from combustion, theload surrounding the heat transfer bed; and adjusting the fluidizationof the heat transfer bed to control the load turndown to control therate of heat transfer to the load.
 2. The method of claim 1 wherein saidfluidized combustion bed and said fluidized heat transfer bed arefluidized from independent fluidization air sources.
 3. The method ofclaim 1 wherein exiting air and gases from the combustion bed and theheat transfer bed are combined.
 4. The method of claim 1 wherein thecombustion bed is separated from the heat transfer bed by a wall.
 5. Themethod of claim 4 wherein the heat transfer bed is defined by the wallbetween the combustion bed and the heat transfer bed and a secondannular wall.
 6. The method of claim 1 wherein each fluidized bedcontains a bed material to be fluidized, said bed material being agranular material.
 7. The method of claim 1 wherein adjusting thefluidization comprises altering the amount of fluidization airintroduced into the heat transfer bed.
 8. The method of claim 1 furthercomprising surrounding at least a portion of the heat transfer bed withat least one water jacket whereby controlled alteration of the loadturndown controls the rate of heat transfer to any water jacket.
 9. Themethod of claim 8 wherein the heat transfer bed and each water jacketare separated by a wall.
 10. A fluidized bed combustor for improving thecontrol of load turndown, the load comprising one or more chambershaving a fluid, comprising:a first chamber defining a fluidizedcombustion bed to contain fuel for combustion bed material to befluidized, and the combustion process; a second chamber that is freefrom combustion defining a separately fluidizable heat transfer bedsurrounding the fluidizable combustion bed and containing bed materialto be fluidized; the load surrounding the heat transfer bed; and so thatby controlling the fluidization of said heat transfer bed, the loadturndown of the fluidized bed combustor can be accurately controlled tocontrol the rate of heat transfer to the load.
 11. The fluidized bedcombustor of claim 10 wherein the first chamber comprises a containerhaving at least continuous side walls and a bottom wall to contain andsupport the bed material and fuel but having means to allow introductionof fluidization air into the interior of the first chamber.
 12. Thefluidized bed combustor of claim 10 wherein the bed material of thecombustion bed and the heat transfer bed is made of a granular material.13. The fluidized bed combustor of bed 10 wherein the second chamber iscomprised of an annular container defined by a wall bordering thecombustion bed, a second spaced-apart annular wall, and a bottom wall tosupport the bed material but having means to allow introduction offluidization air into the interior of the second chamber.
 14. Thefluidized bed combustor of claim 10 wherein the combustion bed and theheat transfer bed have independent fluidization air sources.
 15. Thefluidized bed combustor of claim 10 further comprising at least onewater jacket surrounding at least a portion of the heat transfer bedwhereby controlled alteration of the load turndown controls the rate ofheat transfer to the water jacket.
 16. The fluidized bed combustor ofclaim 10 wherein fluidization of the combustion bed and the heattransfer bed is independently controlled by separately altering theamount of fluidization air introduced into the combustion bed and theheat transfer bed respectively.